WO 02/76925 (Eli Lilly), WO 00/06254 (Societe Civile Bioprojet), WO 01/66534 (Abbott Laboratories) and WO 02/12190 (Ortho McNeil Pharmaceutical Inc) describe a series of compounds which are claimed to be histamine H3 antagonists. WO 02/094788 (Eli Lilly and Company) describe a series of substituted tetrahydroquinoline derivatives which are claimed to be useful in the treatment of diseases associated with aberrant physiological responses to endogenous estrogen. WO 01/23374 (SmithKline Beecham plc) describe a series of piperazine derivatives as 5-HT1B antagonists which are claimed to be useful in the treatment of CNS disorders. WO 98/40385 (Novo Nordisk) describe a series of tetrahydrothienopyridine derivatives which are claimed to be useful in diseases related to glucose metabolic pathways. WO 95/34540 and JP 09221476 (both Otsuka Pharm Co) describe a series of benzoheterocyclic derivatives as vasopressin or oxytocin modulators which are claimed to be useful in a variety of disorders. WO 01/66520 (Ono Pharm Co) describe a series of indole derivatives as prostaglandin D antagonists which are claimed to be useful in allergic diseases, pruritus and cerebrovascular disease.
The histamine H3 receptor is predominantly expressed in the mammalian central nervous system (CNS), with minimal expression in peripheral tissues except on some sympathetic nerves (Leurs et al., (1998), Trends Pharmacol. Sci. 19, 177-183). Activation of H3 receptors by selective agonists or histamine results in the inhibition of neurotransmitter release from a variety of different nerve populations, including histaminergic and cholinergic neurons (Schlicker et al., (1994), Fundam. Clin. Pharmacol. 8, 128-137). Additionally, in vitro and in vivo studies have shown that H3 antagonists can facilitate neurotransmitter release in brain areas such as the cerebral cortex and hippocampus, relevant to cognition (Onodera et al., (1998), In: The Histamine H3 receptor, ed Leurs and Timmerman, pp 255-267, Elsevier Science B.V.). Moreover, a number of reports in the literature have demonstrated the cognitive enhancing properties of H3 antagonists (e.g. thioperamide, clobenpropit, ciproxifan and GT-2331) in rodent models including the five choice task, object recognition, elevated plus maze, acquisition of novel task and passive avoidance (Giovanni et al., (1999), Behav. Brain Res. 104, 147-155). These data suggest that novel H3 antagonists and/or inverse agonists such as the current series could be useful for the treatment of cognitive impairments in neurological diseases such as Alzheimer's disease and related neurodegenerative disorders.